In tissues, for reactions using energy such as biosynthesis, active transport, muscle contraction and the like, the energy is supplied by hydrolysis of adenosine triphosphate (ATP). ATP is produced by oxidation of metabolic fuel which yields much energy, such as glucose and free fatty acids. In oxidative tissues such as muscle, ATP is mostly produced from acetyl-CoA that enters citric acid cycle. Acetyl-CoA is produced by oxidation of glucose via glycolytic pathway or β oxidation of free fatty acid. An enzyme that plays a pivotal role in controlling acetyl-CoA production from glucose is pyruvate dehydrogenase (hereinafter to be abbreviated as PDH). PDH catalyzes reduction of nicotinamide adenine dinucleotide (NAD) to NADH, simultaneously with oxidation of pyruvic acid to acetyl-CoA and carbon dioxide (e.g., non-patent documents 1, 2).
PDH is a multienzyme complex consisting of three enzyme components (E1, E2 and E3) and some subunits localized in mitochondrial matrix. E1, E2 and E3 are responsible for decarboxylation from pyruvic acid, production of acetyl-CoA and reduction of NAD to NADH, respectively.
Two classes of enzyme having regulatory function bind to PDH. One is PDHK, which is a protein kinase having specificity to PDH. The role thereof is to inactivate E1α subunit of the complex by phosphorylation. The other is PDH phosphatase, which is a specific protein phosphatase that activates PDH via dephosphorylation of E1α subunit. The proportion of PDH in its active (dephosphorylated) state is determined by the balance of kinase activity and phosphatase activity. The kinase activity is regulated by the relative concentration of metabolic substrates. For example, the kinase activity is activated by an increase in NADH/NAD, acetyl-CoA/CoA and ATP/adenosine diphosphate (ADP) ratios, and inhibited by pyruvic acid (e.g., non-patent document 3).
In the tissues of mammals, 4 kinds of PDHK isozymes are identified. Particularly, PDHK2 is expressed in a wide range of tissues including the liver, skeletal muscles and adipose tissues involved in glucose metabolism. Furthermore, since PDHK2 shows comparatively high sensitivity to activation by increased NADH/NAD or acetyl-CoA/CoA and inhibition by pyruvic acid, involvement in a short-term regulation of glucose metabolism is suggested (e.g., non-patent document 4).
In addition, PDHK1 is expressed in large amounts in cardiac muscle, skeletal muscle, pancreatic β cell and the like. Furthermore, since expression of PDHK1 is induced via activation of hypoxia inducible factor (HIF) 1 in ischemic state, its involvement in ischemic diseases and cancerous diseases is suggested (e.g., non-patent document 5).
In diseases such as insulin-dependent (type 1) diabetes, non-insulin-dependent (type 2) diabetes and the like, oxidation of lipids is promoted with simultaneous reduction in glucose utilization. This reduction in glucose utilization is one of the factors causing hyperglycemia. When the oxidative glucose metabolism decreases in type 1 and type 2 diabetes and obesity, PDH activity also decreases. It suggests involvement of reduced PDH activity in the reduced glucose utilization in type 1 and type 2 diabetes (e.g., non-patent documents 6, 7).
On the contrary, hepatic gluconeogenesis is enhanced in type 1 and type 2 diabetes, which also forms one factor causing hyperglycemia. The reduced PDH activity increases pyruvic acid concentration, which in turn increases availability of lactic acid as a substrate for hepatic gluconeogenesis. It suggests possible involvement of reduced PDH activity in the enhanced gluconeogenesis in type 1 and type 2 diabetes (e.g., non-patent documents 8, 9). When PDH is activated by inhibition of PDHK, the rate of glucose oxidation is considered to rise. As a result, glucose utilization in the body is promoted and hepatic gluconeogenesis is suppressed, whereby hyperglycemia in type 1 and type 2 diabetes is expected to be improved (e.g., non-patent documents 10, 11, 12). Another factor contributing to diabetes is impaired insulin secretion, which is known to be associated with reduced PDH activity in pancreatic β cells, and introduction of PDHK1, 2 and 4 (e.g., non-patent documents 13, 14). In addition, sustained hyperglycemia due to diabetes is known to cause complications such as diabetic neuropathy, diabetic retinopathy, diabetic nephropathy and the like. Thiamine and α-lipoic acid contribute to activation of PDH as coenzymes. Thiamine and α-lipoic acid, or thiamine derivative and α-lipoic acid derivative are shown to have a promising effect on the treatment of diabetic complications. Thus, activation of PDH is expected to improve diabetic complications (e.g., non-patent documents 15, 16).
Under ischemic conditions, limited oxygen supply reduces oxidation of both glucose and fatty acid and reduces the amount of ATP produced by oxidative phosphorylation in the tissues. In the absence of sufficient oxygen, ATP level is maintained by promoted anaerobic glycolysis. As a result, lactic acid increases and intracellular pH decreases. Even though the body tries to maintain homeostasis of ion by energy consumption, abnormally low ATP level and disrupted cellular osmolarity lead to cell death. In addition, adenosine monophosphate-activating kinase, activated during ischemia, phosphorylates and thus inactivates acetyl-CoA carboxylase. The levels of total malonyl-CoA in the tissue drop, carnitine palmitoyltransferase-I activity is therefore increased and fatty acid oxidation is favored over glucose oxidation by allowing the transport of acyl-CoA into mitochondria. Oxidation of glucose is capable of yielding more ATP per molecule of oxygen than is oxidation of fatty acids. Under ischemic conditions, therefore, when energy metabolism becomes glucose oxidation dominant by activation of PDH, the ability to maintain ATP level is considered to be enhanced (e.g., non-patent document 17).
In addition, since activation of PDH causes oxidation of pyruvic acid produced by glycolysis, and reducing production of lactic acid, the net proton burden is considered to be reduced in ischemic tissues. Accordingly, PDH activation by inhibition of PDHK is expected to protectively act in ischemic diseases such as cardiac muscle ischemia (e.g., non-patent documents 18, 19).
A drug that activates PDH by inhibition of PDHK is considere d to decrease lactate production since it promotes pyruvate metabo lism. Hence, such drug is expected to be useful for the treatment of hyperlactacidemia such as mitochondrial disease, mitochondrial encephalomyopathy and sepsis (e.g., non-patent document 20).
In cancer cells, the expression of PDHK1 or 2 increases. In cancer cells, moreover, ATP production by oxidative phosphorylation in mitochondria decreases, and ATP production via the anaerobic glycolysis in cytoplasm increases. PDH activation by inhibition of PDHK is expected to promote oxidative phosphorylation in mitochondria, and increase production of active oxygen, which will induce apoptosis of cancer cells. Therefore, the PDH activation by PDHK inhibition is useful for the treatment of cancerous diseases (e.g., non-patent document 21).
Pulmonary hypertension is characterized by high blood pressure caused by partial narrowing of the pulmonary artery due to promoted cell proliferation therein. In pulmonary hypertension, therefore, activation of PDH in the pulmonary artery cell is expected to promote oxidative phosphorylation in mitochondria, increase production of active oxygen, and induce apoptosis of the pulmonary artery cells. Therefore, the PDH activation by PDHK inhibition is considered to be useful for the treatment of pulmonary hypertension (e.g., non-patent document 22).
Energy production and glucose metabolism in the cerebrum decrease in Alzheimer disease, and also, PDH activity declines. When the PDH activity declines, production of acetyl CoA decreases. Acetyl CoA is utilized for ATP production in the electron transport system via the citric acid cycle. Acetyl CoA is also a starting material for synthesizing acetylcholine, which is one of the neurotransmitters. Therefore, reduced brain PDH activity in Alzheimer disease is considered to cause neuronal cell death due to the decreased ATP production. Moreover, it is considered that synthesis of acetylcholine, which is the transmitter for cholinergic nerve, is inhibited to induce deterioration of memory and the like. Activation of PDH in the brain is expected to enhance energy production and acetylcholine synthesis in Alzheimer disease. Therefore, activation of PDH by the inhibition of PDHK is considered to be useful for the treatment of Alzheimer disease (e.g., non-patent documents 23, 24).
It has been shown that dichloroacetic acid, which is a drug having a PDH activating action, provides promising effects for the treatment of diabetes, myocardial ischemia, myocardial infarction, angina pectoris, cardiac failure, hyperlactacidemia, brain ischemia, cerebral apoplexy, peripheral arterial disease, chronic obstructive pulmonary disease, cancerous disease, and pulmonary hypertension (e.g., non-patent documents 10, 18, 20, 22, 25, 26, 27).
From the foregoing findings, a PDHK inhibitor is considered to be useful for the prophylaxis or treatment of diseases relating to glucose utilization disorder, for example, diabetes (type 1 diabetes, type 2 diabetes etc.), insulin resistance syndrome, metabolic syndrome, hyperglycemia, hyperlactacidemia, diabetic complications (diabetic neuropathy, diabetic retinopathy, diabetic nephropathy, cataract etc.). Furthermore, a PDHK inhibitor is considered to be useful for the prophylaxis or treatment of diseases caused by limited energy substrate supply to the tissues, for example, cardiac failure (acute cardiac failure, chronic cardiac failure), cardiomyopathy, myocardial ischemia, myocardial infarction, angina pectoris, dyslipidemia, atherosclerosis, peripheral arterial disease, intermittent claudication, chronic obstructive pulmonary disease, brain ischemia and cerebral apoplexy.
Therefore, a PDHK inhibitor is considered to be useful for the treatment or prophylaxis of diabetes (type 1 diabetes, type 2 diabetes etc.), insulin resistance syndrome, metabolic syndrome, hyperglycemia, hyperlactacidemia, diabetic complications (diabetic neuropathy, diabetic retinopathy, diabetic nephropathy, cataract etc.), cardiac failure (acute cardiac failure, chronic cardiac failure), cardiomyopathy, myocardial ischemia, myocardial infarction, angina pectoris, dyslipidemia, atherosclerosis, peripheral arterial disease, intermittent claudication, chronic obstructive pulmonary disease, brain ischemia, cerebral apoplexy, mitochondrial disease, mitochondrial encephalomyopathy, cancer, pulmonary hypertension or Alzheimer disease.